In a large skillet over med-high heat, crisp the pancetta until lightly golden. Add onions and cook until they begin to soften, about 5 minutes. Add the mushrooms and cook until they are soft and moisture is absorbed. Remove from heat and stir in the thyme and pepper. Allow to cool, about 8 minutes.

In a small bowl whisk eggs and cream together. Sprinkle the bottom of the pie crust with cheese. Evenly spread the mushroom mixture over the cheese. Gently pour the cream on top.

Bake 35 - 45 minutes, until the custard is set and top is golden. Allow quiche to rest 10 minutes before slicing into wedges.